1. Field of the Invention:
This invention is related generally to power supplies and specifically to battery charging systems for continuously charging a back-up battery.
2. Description of the Prior Art:
In many applications it is desirable to maintain the operation of equipment even if the main power source should fail. One such application is the measurement by a utility of electrical energy consumption. In this application, an event-logging system must remain operational in the event of a power failure so that the system clock can maintain correct real time information and the event-logging system can record information about the power outage itself. A battery back-up, or carry-over, is typically provided for this purpose.
In an event-logging system it is probable that the back-up battery will not be used for extended periods of time. However, since it is not known when the back-up battery will be needed, the battery should be fully charged and available at all times. One method of assuring the availability of the battery is to continuously charge the battery. The type of battery normally chosen to provide the battery back-up for an event-logging system is a nickel-cadmium battery.
Nickel-cadmium batteries are available in a variety of constructions and are the most widely-used of the small, sealed or maintenance free rechargeable batteries. However, the charging reaction of a nickel-cadmium battery is extremely temperature sensitive. At or above room temperature, 25.degree. C., oxygen produced by the charging reaction chemically recombines thereby preventing the build-up of pressure. Below room temperature the oxygen does not recombine as readily thereby allowing pressure to build up. As pressure builds up the danger of venting is increased. Consequently, it is desirable to decrease the charge rate, or stop charging altogether, as the temperature drops.
Additionally, as the temperature increases, the charging efficiency decreases. At room temperature the charging efficiency is approximately 85%. At 80.degree. C. the efficiency is approximately 40%. Thus, as the temperature increases, the charging current must increase in order to overcome the decrease in efficiency of the charging reaction.